<p>This paper carries out a comprehensive and systematic comparison study on the kinematic performance of four six degrees of freedom (6-DOF) parallel mechanisms with different topologies, i.e., 6-U<Emphasis Type="Underline">P</Emphasis>S, 3-(2-U<Emphasis Type="Underline">P</Emphasis>R)U, 3-(2-U<Emphasis Type="Underline">C</Emphasis>R)U, and 3-R(2-R<Emphasis Type="Underline">P</Emphasis>R)U. The research begins by elaborating in detail the similarities and differences among these four parallel mechanisms. By standardizing the definition of the coordinate system for each mechanism, the inverse kinematics and the Jacobian matrix of these four mechanisms are systematically derived. Employing a set of motion/force transmission indices, which are directly obtained from the Jacobian matrix, the kinematic performances of the four mechanisms are thoroughly analyzed and compared within the given workspaces while maintaining the same dimensional parameters for all cases. The comparison study of these four parallel mechanisms extends beyond local transmission indices to also include global transmission indices, covering both position and orientation workspaces, as well as assessments at both the local and global workspace levels. This comprehensive approach ensures a detailed and fair evaluation of their respective kinematic capabilities. The results indicate that the comprehensive kinematic performances of the four parallel mechanisms are similar, and providing a solid theoretical foundation for innovative design and practical guidance for selecting optimal architectures based on specific application requirements.</p>